Phosphine-Catalyzed Synthesis and Cytotoxic Evaluation of Michael Adducts of the Sesquiterpene Lactone Arglabin.

A general method for chemo- and diastereoselective modification of anticancer natural product arglabin with nitrogen- and carbon-centered pronucleophiles under the influence of nucleophilic phosphine catalysts was developed. The locked s-cis-geometry of α-methylene-γ-butyrolactone moiety of arglabin favors for the additional stabilization of the zwitterionic intermediate by electrostatic interaction between phosphonium and enolate oxygen centers, leading to the unprecedentedly high efficiency of the phosphine-catalyzed Michael additions to this sesquiterpene lactone. Using n-Bu3P as the catalyst, pyrazole, phthalimide, 2-oxazolidinone, 4-quinazolinone, uracil, thymine, cytosine, and adenine adducts of arglabin were obtained. The n-Bu3P-catalyzed reaction of arglabin with active methylene compounds resulted in the predominant formation of bisadducts bearing a new quaternary carbon center. All synthesized Michael adducts and previously obtained phosphorylated arglabin derivatives were evaluated in vitro against eleven cancer and two normal cell lines, and the results were compared to those of natural arglabin and its dimethylamino hydrochloride salt currently used as anticancer drugs. 2-Oxazolidinone, uracil, diethyl malonate, dibenzyl phosphonate, and diethyl cyanomethylphosphonate derivatives of arglabin exhibited more potent antiproliferative activity towards several cancer cell lines and lower cytotoxicity towards normal cell lines in comparison to the reference compounds, indicating the feasibility of the developed methodology for the design of novel anticancer drugs with better therapeutic potential.

Synthesis and antihepatoma activity of guaianolide dimers derived from lavandiolide I.

Guaianolide dimers represent a unique class of natural products with anticancer activities, but their low content in plants has limited in-depth pharmacological studies. Lavandiolide I is a guaianolide dimer isolated from Artemisia species, and had been synthesized on a ten-gram scale in four steps with 60 % overall yield, which showed potent antihepatoma activity on the HepG2, Huh7, and SK-Hep-1 cell lines with IC50 values of 12.1, 18.4, and 17.6 µM, respectively. To explore more active dimers, 33 lavandiolide I derivatives were designed, synthesized, and evaluated for their inhibitory activity on human hepatoma cell lines. Among them, 10 derivatives were more active than lavandiolide I and sorafenib on the three cell lines. The primary structure-activity relationship concluded that the introduction of aldehyde, ester, azide, amide, carbamate and urea functional groups at C-14' of the guaianolide dimer significantly enhanced the antihepatoma activity. Among these compounds, derivatives 25, 27, and 33 enhanced antihepatoma activity more than 1.2-5.8 folds than that of lavandiolide I, and demonstrated low toxicity to the human liver cell lines (THLE-2) and good safety profiles with selective index ranging from 1.3 to 3.4, while lavandiolide I was more toxic to THLE-2 cells. This work provides new insights into enhancing the antihepatoma efficacy and reducing the toxicity of sesquiterpenoid dimers.

Design, synthesis and in vivo anticancer activity of novel parthenolide and micheliolide derivatives as NF-κB and STAT3 inhibitors.

Parthenolide and micheliolide have attracted great attention in anticancer research due to their unique activities. In this study, thirteen parthenolide derivatives and twenty-three micheliolide derivatives were synthesized. Most synthesized compounds showed higher cytotoxicity than parthenolide or micheliolide. The in vivo anticancer activity of several representative compounds was evaluated in mice. One micheliolide derivative, 9-oxomicheliolide (43), showed promising in vivo antitumor activity compared with clinical drugs cyclophosphamide or temozolomide. Compound 43 was particularly effective against glioblastoma, with its tumor inhibition rate in mice comparable to the drug temozolomide. The discovery of compound 43 also demonstrates the feasibility of developing anticancer micheliolide derivatives by modification at C-9 position. Anticancer mechanism studies revealed that 9-oxomicheliolide exhibited inhibition effect against NF-κB and STAT3 signaling pathways, as well as induction effects of cell apoptosis. It is postulated that 9-oxomicheliolide is likely to be a modulator of the immune system, which regulates the anticancer immune responses.

Synthesis of Guaianolide Analogues with a Tunable α-Methylene-γ-lactam Electrophile and Correlating Bioactivity with Thiol Reactivity.

α-Methylene-γ-lactones are present in ∼3% of known natural products, and compounds comprising this motif display a range of biological activities. However, this reactive lactone limits informed structure-activity relationships for these bioactive molecules. Herein, we describe chemically tuning the electrophilicity of the α-methylene-γ-lactone by replacement with an α-methylene-γ-lactam. Guaianolide analogues having α-methylene-γ-lactams are synthesized using the allenic Pauson-Khand reaction. Substitution of the lactam nitrogen with electronically different groups affords diverse thiol reactivity. Cellular NF-κB inhibition assays for these lactams were benchmarked against parthenolide and a synthetic α-methylene-γ-lactone showing a positive correlation between thiol reactivity and bioactivity. Cytotoxicity assays show good correlation at the outer limits of thiol reactivity but less so for compounds with intermediate reactivity. A La assay to detect reactive molecules by nuclear magnetic resonance and mass spectrometry peptide sequencing assays with the La antigen protein demonstrate that lactam analogues with muted nonspecific thiol reactivities constitute a better electrophile for rational chemical probe and therapeutic molecule design.

Antitumor Effects and Tumor-specificity of Guaiazulene-3-Carboxylate Derivatives Against Oral Squamous Cell Carcinoma In Vitro.

AIM: The aim of this study was to investigate the antitumor potential of guaiazulene-3-carboxylate derivatives against oral malignant cells. MATERIALS AND METHODS: Twelve guaiazulene-3-carboxylate derivatives were synthesized by introduction of either with alkyl group [1-5], alkoxy group [6, 7], hydroxyl group [8, 9] or primary amine [10-12] at the end of sidechains. Tumor-specificity (TS) was calculated by the ratio of mean 50% cytotoxic concentration (CC50) against 3 human oral mesenchymal cell lines to that against 4 human oral squamous cell carcinoma (OSCC) cell lines. Potency-selectivity expression (PSE) was calculated by dividing TS value by CC50value against OSCC cell lines. Cell cycle analysis was performed by cell sorter. RESULTS: [6, 7] showed the highest TS and PSE values, and induced the accumulation of both subG1 and G2/M cell populations in HSC-2 OSCC cells. Quantitative structure-activity relationship analysis demonstrated that their tumor-specificity was correlated with chemical descriptors that explain the 3D shape, electric state and ionization potential. CONCLUSION: Alkoxyl guaiazulene-3-carboxylates [6, 7] can be potential candidates of lead compound for developing novel anticancer drugs.

Shape Similarity by Fractal Dimensionality: An Application in the de novo Design of (-)-Englerin A Mimetics.

Molecular shape and pharmacological function are interconnected. To capture shape, the fractal dimensionality concept was employed, providing a natural similarity measure for the virtual screening of de novo generated small molecules mimicking the structurally complex natural product (-)-englerin A. Two of the top-ranking designs were synthesized and tested for their ability to modulate transient receptor potential (TRP) cation channels which are cellular targets of (-)-englerin A. Intracellular calcium assays and electrophysiological whole-cell measurements of TRPC4 and TRPM8 channels revealed potent inhibitory effects of one of the computer-generated compounds. Four derivatives of this identified hit compound had comparable effects on TRPC4 and TRPM8. The results of this study corroborate the use of fractal dimensionality as an innovative shape-based molecular representation for molecular scaffold-hopping.

Syntheses of Epoxyguaiane Sesquiterpenes (-)-Englerin A, (-)-Oxyphyllol, (+)-Orientalol E, and (+)-Orientalol F: A Synthetic Biology Approach.

A combined approach toward syntheses of epoxyguaiane sesquiterpenes is presented. By use of a fungus sesquiterpene cyclase, guaian-6,10(14)-diene was produced through metabolic engineering of the isoprenoid pathway in E. coli. (-)-Englerin A, (-)-oxyphyllol, (+)-orientatol E, and (+)-orientalol F have been synthesized in two to six steps. This strategy provided rapid access to the epoxyguaiane core structure and would facilitate syntheses of (-)-englerin A and its analogues for evaluation of their therapeutic potentials in drug discovery.

Semisynthesis of Plant-Derived Englerin A Enabled by Microbe Engineering of Guaia-6,10(14)-diene as Building Block.

Herein, we report a short semisynthesis of the potent transient receptor potential canonical (TRPC) channel agonist englerin A (EA) and the related guaianes oxyphyllol and orientalol E. The guaia-6,10(14)-diene starting material was systematically engineered in Escherichia coli and Saccharomyces cerevisiae using the CRISPR/Cas9 system and was produced with high titers. The potentially scalable approach combines the advantages of synthetic biology and chemical synthesis providing an efficient and economical method for producing EA and analogues.

Allylative Approaches to the Synthesis of Complex Guaianolide Sesquiterpenes from Apiaceae and Asteraceae.

With hundreds of unique members isolated to date, guaianolide lactones represent a particularly prolific class of terpene natural products. Given their extensive documented therapeutic properties and fascinating chemical structures, these metabolites have captivated the synthetic chemistry community for many decades. As a result of divergent biosynthetic pathways, which produce a wide array of stereochemical and oxidative permutations, a unifying synthetic pathway to this broad family of natural products is challenging. Herein we document the evolution of a chiral-pool-based synthetic program aimed at accessing an assortment of guaianolides, particularly those from the plant family Apiaceae as well as Asteraceae, members of which possess distinct chemical substructures and necessitate deviating synthetic platforms. An initial route employing the linear monoterpene linalool generated a lower oxidation state guaianolide but was not compatible with the majority of family members. A double-allylation disconnection using a carvone-derived fragment was then developed to access first an Asteraceae-type guaianolide and then various Apiaceae congeners. Finally, using these findings in conjunction with a tandem polyoxygenation cascade, we developed a pathway to highly oxygenated nortrilobolide. A variety of interesting observations in metal-mediated aldehyde allylation and alkene polyoxygenation are reported and discussed.

ß-Ketoesters as Mono- or Bisnucleophiles: A Concise Enantioselective Total Synthesis of (-)-Englerin†A and B.

A short enantioselective total synthesis of englerin A, a guaiane sesquiterpene with significant in vitro antitumor activity, is reported. Key features of this total synthesis are an organocatalytic asymmetric decarboxylative aldol reaction, a neighboring-group-participating [4+3] cycloaddition, a novel one-pot Heck coupling/regioselective 1,4-hydrosilylation/Tamao-Fleming oxidation cascade, and a kinetic CBS reduction, generating the optically pure natural product in 6.7 % overall yield over twelve steps starting from methylglyoxal. Selective saponification of the more reactive glycolic ester moiety of englerin A also gave (-)-englerin B.

[Stereocontrolled Total Synthesis of Biologically Active Natural Products].

　This review article describes the total syntheses of englerin A, ophiodilactones A and B, marinomycin A, N-methylwelwitindolinone C isothiocyanate, tirandamycins A-D, and tirandalydigin, which possess intriguing biological activities and challenging structures with characteristic ring systems. The focus is on the synthetic methodologies that lead to the highly stereocontrolled assembly of these natural products.

Formal synthesis of englerin A utilizing regio- and diastereoselective [4+3] cycloaddition.

Englerin A, a guaiane sesquiterpene isolated from Phyllanthus engleri, showed highly potent and selective growth inhibitory activities against renal cancer cell lines. We synthesized the key tricyclic intermediate from commercially available 2,2-dimethyl-1,3-dioxan-5-one via regio- and diastereoselective [4+3] cycloaddition between the formyl enol silyl ether and the disubstituted furan, in 4.8% total yield over 10 steps.

Unified total synthesis of (+)-chinensiolide B and (+)-8-epigrosheimin.

An expedient synthetic approach has been developed for the unified total synthesis of (+)-chinensiolide B and (+)-8-epigrosheimin. The point of divergence was provided by the lactone aldehyde 6, in which four contiguous stereocenters were achieved by a stereocontrolled Evans syn-aldol reaction of a R-carvone derived enantiopure aldehyde and chiral N-succinyl-oxazolidinone. The lactone aldehyde 6 was synthesized in multigram quantity in three steps. Highly optimized chemo- and stereoselective reactions and functional group interconversion enabled us to assemble (+)-chinensiolide B and (+)-8-epigrosheimin from 6.

A Concise, Efficient and Scalable Total Synthesis of Thapsigargin and Nortrilobolide from (R)-(-)-Carvone.

A concise, efficient and scalable synthesis of thapsigargin and nortrilobolide from commercially available (R)-(-)-carvone was developed. Our synthetic strategy is inspired by nature's carbon-carbon bond formation sequence, which facilitates the construction of a highly functionalized sesquiterpene lactone skeleton in five steps via an enantioselective ketone alkylation and a diastereoselective pinacol cyclization. We envision that this strategy will permit the construction of other members of the family, structural analogs and provide a practical synthetic route to these important bioactive agents. In addition, we anticipate that the prodrug Mipsagargin, which is currently in late-stage clinical trials for the treatment of cancer, will also be accessible via this strategy. Hence, the limited availability from natural sources, coupled with an estimated demand of one metric ton per annum for the prodrug, provides a compelling mandate to develop practical total syntheses of these agents.

A Double Allylation Strategy for Gram-Scale Guaianolide Production: Total Synthesis of (+)-Mikanokryptin.

With over 5000 members isolated to date, sesquiterpene lactones represent a prolific source of medicinal agents with several derivatives in human clinical trials. The guaianolides, a major subset of this group, have been intensely investigated from both medicinal and chemical-synthesis perspectives for decades. To date, the myriad stereochemical permutations presented by this enormous family have precluded the synthesis of many unique members. Herein we report the total synthesis of the trans-fused 8,12-guaianolide (+)-mikanokryptin in 10 steps from (+)-carvone. Notably, this synthesis is the first gram-scale total synthesis of a guaianolide natural product.

Asymmetric total synthesis of hedyosumin E aglycon, 7,10-epoxyhedyosminolide and ent-zedolactone A.

The asymmetric total syntheses of hedyosumin E aglycon, 7,10-epoxyhedyosminolide and ent-zedolactone A were realized, with the first two being achieved for the first time.

Synthesis and Biological Evaluation of New (-)-Englerin Analogues.

We report the synthesis and biological evaluation of a series of (-)-englerin A analogues obtained along our previously reported synthetic route based on a stereoselective gold(I) cycloaddition process. This synthetic route is a convenient platform to access analogues with broad structural diversity and has led us to the discovery of unprecedented and easier-to-synthesize derivatives with an unsaturation in the cyclopentyl ring between C4 and C5. We also introduce novel analogues in which the original isopropyl motif has been substituted with cyclohexyl, phenyl, and cyclopropyl moieties. The high selectivity and growth-inhibitory activity shown by these new derivatives in renal cancer cell lines opens new ways toward the final goal of finding effective drugs for the treatment of renal cell carcinoma (RCC).

Catalytic Asymmetric Total Synthesis of Hedyosumins A, B, and C.

The first and asymmetric total synthesis of hedyosumins A, B, and C was accomplished in 13-14 steps from simple starting materials. The essential tools that allow us to access the tetracyclic skeleton include an organocatalytic [4 + 3] cycloaddition reaction, an intramolecular aldol condensation, and an intramolecular carboxymercuration/demercuration enabled lactonization. A CBS-catalyzed asymmetric reduction was employed to boost the ee of the synthetic natural products to an excellent level. This synthesis established the absolute configurations of hedyosumins A, B, and C.

Total Synthesis of (±)-Englerin†A Using An Intermolecular [3+2] Cycloaddition Reaction of Platinum-Containing Carbonyl Ylide.

Total synthesis of (±)-Englerin A has been achieved starting from γ,δ-ynone 5 in 14 steps. The key feature of this synthesis is the highly efficient and stereoselective preparation of 8-oxabicyclo[3.2.1]octane derivative 6, a core skeleton of Englerin A, based on an inverse electron-demand [3+2] cycloaddition reaction of the platinum-containing carbonyl ylide, which was developed in our laboratory.

Asymmetric Total Synthesis of (-)-Englerin A through Catalytic Diastereo- and Enantioselective Carbonyl Ylide Cycloaddition.

An asymmetric total synthesis of the guaiane sesquiterpene (-)-englerin A, a potent and selective inhibitor of the growth of renal cancer cell lines, was accomplished. The basis of the approach is a highly diastereo- and enantioselective carbonyl ylide cycloaddition with an ethyl vinyl ether dipolarophile under catalysis by dirhodium(II) tetrakis[N-tetrachlorophthaloyl-(S)-tert-leucinate], [Rh2 (S-TCPTTL)4 ], to construct the oxabicyclo[3.2.1]octane framework with concomitant introduction of the oxygen substituent at C9 on the exo-face. Another notable feature of the synthesis is ruthenium tetraoxide-catalyzed chemoselective oxidative conversion of C9 ethyl ether to C9 acetate.